The helix resonator is a transmission line resonator with a physical length of about a quarter of wavelength. The resonator comprises inductive elements consisting of a conductor wound into a cylindrical coil and encapsulated by a metallic housing spaced apart therefrom. The low impedance (grounded) end of the coil can be connected directly to the metallic housing, and the opposite end, a high-impedance end, is spaced away from the housing and capacitatively coupled thereto.
The characteristic impedance of the helix resonator is determined by the ratio of the coil diameter and the inner dimensions of the encapsulating housing, by the distance of the turns of the coil from each other, i.e. by the so-called pitch, and possibly, by the insulating material supporting the resonator. The resonant frequency of the helix resonator is a function of the physical properties of the coil, the capacitative structure, and the distance of the high-impedance end from the housing. Therefore, in order to produce a resonator of a given frequency band, a precise and exact structure is required.
By electromagnetically coupling resonators together, a filter provided with desired properties can be constructed. In practice, this is accomplished by the resonator coils being inserted in one and same housing and having a partition disposed between individual resonators. The size of any apertures in the partition determines the electromagnetic coupling between the resonators.
As mentioned above, the resonator coil can be mechanically supported and attached via the insulating material to the housing. The support can comprise injection moulded plastic bonds, which on one side are bound on the wall of the housing and on another side contact a few rotations of the resonator. Also a cylindrical insulating body can be used, around which the conducting wire of the resonator may be wound. Finnish patent FI-78198 discloses a helix resonator in which the resonator coil has been supported with an insulation plate, on which an electrical circuit made from strip lines has moreover been disposed, to which circuit the resonator has been coupled electrically. Said construction which forms the starting point for the present application, is presented in FIGS. 1 and 2. The four-circuit filter construction presented therein comprises four discrete helix resonators 1 wound from metal wire into a cylindrical coil. Each resonator has been fitted around the finger-resembling projections 2a of the plate 2 made from an insulating material. The construction is known in the art as a comb structure. In the lower part of the insulation plate an electric circuit can be produced from strip lines 3, to which the resonator is coupled e.g. by soldering at points indicated by reference numerals 4. Each resonator has also been at the upper end attached to projection 2a by soldering it to the metallized point in the projection. Such points of juncture are indicated in FIG. 1 by reference numeral 5. In the upper edge of each projection 2a and in the ends of the lower part of the insulation plate there is provided a foil strip 6 for soldering the insulation plate to the housing. The projection is soldered to the cover using a manner described below.
The housing, shown in FIG. 3, is an elongate extruded box, having an upper surface 8 and four side surfaces, and three partitions, of which walls 9 and 12 are shown. Each partition is provided with a slit 10 extending upwards from the lower edge, the length thereof being the same as the height P of the integral lower part of the circuit board. In this manner four compartments are produced. The circuit board with the resonators thereon is inserted into the housing so that each resonator enters its individual compartment. The circuit board intrudes into the slits in the partitions and the tips of the finger-resembling projections 2a enter the apertures 11 made on the cover of the housing. The ends 7,7' of the lower part of the circuit board enter the grooves made in the end walls of the housing. In this manner the circuit board is supported by the ends, the tips of the finger-resembling projections and at three points in the middle to the housing. The final fixing is done by soldering the foil strip 6 at the tips of the projections (FIG. 1) onto the housing cover, and the ends 7,7' of the circuit board at the equivalent foil strips to the end walls of the housing. Finally, a bottom plate can be fixed, whereby the entire structure becomes encapsulated.
The end result is shown in FIG. 3 in which the housing is partly sectioned for the sake of clarity. Merely the tips of the projections and the end surfaces 7 of the lower part of the board are visible of the circuit board.
Below, a closer look is taken on how in a state of the art structure the projections have been supported by and connected to the cover of the housing. The method is shown by FIGS. 4,5 and 6. FIG. 4 shows a cross-sectional view B--B of the filter shown in FIG. 3, FIG. 5 shows a top view of the supporting point, and FIG. 6 shows a cross-sectional view in the longitudinal direction of the housing. FIG. 5 shows that on the cover of housing 6 a T-shaped indentation 13 has been formed, the transversal part thereof being substantially equal to the broad dimension of projection 2a, i.e. the thickness of the circuit board and the width of the projection. Thus, the tip of the projection 2a enters that part of the indentation. The longitudinal part of the T indentation serves as the exit for surplus soldering paste when the projection is soldered on to the cover of the housing. The indentation may also be rectangular in shape if the discharge of paste has otherwise been addressed. After forming the indentation, the area around the punching point is depressed with a round-ended stick placed perpendicularly against the surface of the housing so that the edges of the area around the punching point bend somewhat inwards into the housing. The line along which the surface of the housing is depicted by broken line L in FIG. 5, and the bending is clearly visible in FIGS. 4 and 6. The conical depression found by the bending facilitates guiding the projection 2a of the circuit board into the indentation, thus improving the soldering of the projection onto the edge of the indentation.
The fixing operation described above involves a number of drawbacks. Firstly, punching the upper surface of the housing is an additional and slow work phase. The punching is accomplished for a large series of housings. Since even a minor error in positioning the punching point greatly affects the properties of the finished filter, endeavours must be made to keep the punching points identical from one housing to another. In practice, this is difficult to maintain. Secondly, when the circuit board is being inserted into the housing, and the tips of the projections intrude into the indentations of the housing cover, it often happens that the sides of the projections become abraded against the edges of the indentations and the soldering foil on the tips get rolled off from the surface of the board. Thus, soldering is no longer so successful as required, thus resulting in a rejected filter.